1. Field of the Invention
The present invention relates generally to optical receivers, and more particularly, to an optical receiver that adjusts the output swing in response to a change in the signal input, with high sensitivity and over a wide dynamic range.
2. Description of the Related Art
FIG. 1 shows a conventional optical receiver used in an optical transmission system for a main line having a maximum transmission distance of 100 km, for example. A receiving element (for example, an avalanche photodiode APD) receives an input optical signal, and outputs an optical ON/OFF rectangular pattern (corresponding to the binary signals "1" and "0", respectively) for every time period .DELTA.t, at a data rate of 1 bit. FIGS. 2(a)-2(c) illustrate a decision operation of the optical receiver for 6 bits of the input optical signal.
In the APD, an electron-hole pair induces avalanche multiplication, as caused by the optical signal, to amplify the signal current (FIG. 2(a)). This amplification makes it possible to detect even a weak optical signal. The current signal output from the APD is converted to a voltage signal VPRE by a preamplifier PRE (FIG. 2(b)). This signal VPRE is further amplified by an automatic-gain-control amplifier AGC (also known as a "gain-controllable amplifier"), and the resulting signal VAGC is input to a decision circuit DEC (FIG. 2(c)).
Even if the intensity of the input optical signal is weak (for example, as illustrated by the broken curve in FIG. 2(a)), the swing of the output signal VAGC of the AGC amplifier is held at a substantially constant level by the automatic gain control function of the AGC amplifier. As a result, the decision made by the decision circuit is stabilized.
In the decision circuit DEC, the datq are output in synchronism with a clock signal CLK by deciding that the output signal VAGC of the AGC amplifier is high ("1") if the output signal VAGC is higher than a reference voltage VTH (also called VTH(DEC) below), or low ("0") if lower than the reference voltage VTH. The clock signal CLK may be prepared from the output signal VAGC by a clock-extraction circuit CEXT.
An example of such an AGC amplifier is disclosed on pp. 815-822 of IEEE Journal of Solid-State Circuits, Vol. 29, No. 7 (1994), and illustrated in the present FIG. 3. A voltage signal V.sub.PRE from the preamplifier PRE is amplified by three stages of amplifiers, including gain-controllable amplifiers A1 and A2, and a constant-gain amplifier A3. Then, the output of the constant-gain amplifier A3, which has a swing V.sub.A3, is input to a peak detector PD. This peak detector PD generates a DC voltage V3 based upon the swing V.sub.A3 using a capacitor C.sub.PD.
A reference circuit REF generates the nominal value of the peak voltage VN (the "nominal voltage swing"). This nominal value VN is adjusted by a variable voltage source P connected to the reference circuit REF.
In the gain control circuit GC, the voltage V3 and the nominal value VN are compared so that voltages V.sub.GC1 and V.sub.GC2 for controlling the gains of the amplifiers A1 and A2 are set according to the difference (i.e., the deviation of the signal swing from the nominal voltage swing). Thus, the output swing of V.sub.AGC from the swing change of the input signal V.sub.PRE is suppressed to the end of obtaining a uniform swing according to the difference in circuit construction between amplifiers A1 and A2.
According to the AGC amplifier of FIG. 3, a voltage signal VAGC having a swing of 500 mV is output at a rate of 13 Gb/sec for an input signal having a swing of either 10 mV or 300 mV. These output signals are output from two output buffers OB so as to be input to the decision circuit DEC and the clock-extraction circuit respectively.
The AGC amplifier of FIG. 3 is further equipped with an offset-control circuit OC (having external capacitors C.sub.OC) for controlling the offsets of the amplifiers A1, A2, and A3.
In response to the output signal from the AGC amplifier, the decision circuit DEC decides whether the signal V.sub.AGC is "1" or "0", by employing a voltage between the upper and lower ends of the swing voltage of the output signal as the reference voltage VTH(DEC).
FIG. 4 illustrates how the optical signal, as transmitted from the optical transmission passage (optical fibers, for example), is converted to a voltage signal by the preamplifier PRE. The abscissa indicates the signal voltage value, and the ordinate indicates the frequency with which the signal is generated at a predetermined voltage level. When the signals are transferred in response to the ON/OFF of the optical signal, the distribution is made, as illustrated by the solid curve in FIG. 4, with noise produced in the photoelectric converting element and in the preamplifier PRE. At this time, the reference voltage set by the decision circuit DEC is set to an intermediate value V.sub.th (A) between the average voltage value V1 of a high signal ("1") and average value V0 of a low signal ("0")
The "1" signal, however, may have a wider voltage distribution than that of the "0" signal. Specifically, the voltage distribution of a "1" signal takes the shape (illustrated by a broken curve in FIG. 4) in which the solid curve of FIG. 4 is depressed in the ordinate direction. This shape is especially prominent when either an APD or an erbium-doped fiber amplifier (EDFA) in combination with a PIN junction photodetector (or PIN diode) is used as the optical receiving element. See "Optical Fiber Communication Technique", published by NIKKAN KOGYO SHINBUN SHA. Voltages signals that do not exceed the reference voltage V.sub.th (A) are decided to be "0" signals even if they are "1" signals in fact, due to this error. The probability for these erroneous decisions is the "error rate" of the decision circuit. For the reference voltage V.sub.th (A), the error rate is defined as the ratio of the area of the region under the broken-line curve to the left of V.sub.th (A), to the entire area under the broken-line curve, which represents the voltage distribution of the actual "1" signal.
The error rate cannot be zero when the voltage distribution curve (broken curve) of the "1" signals and the voltage distribution curve (solid curve) of the "0" signals overlap. However, the error rate can be reduced when the threshold voltage is considerably shifted toward V.sub.0. For example, a threshold voltage V.sub.th (B) can be set at a point where the voltage distribution curve of the "1" signal and the voltage distribution curve of the "0" signal intersect. Hence, although not eliminated, the error rate can be reduced as low as the area ratio shown hatched in FIG. 4.
Although some actual "0" signals (having an intensity over Vth(B)) are detected as "1" signals in the decision circuit due to an error component under Vth(B), the total error rate is reduced compared with that under Vth(A). Moreover, the distribution of the signal voltage extends wider than the magnitude (V1-V0) of the signal anyway, so that the error rate cannot be completely reduced to 0 irrespective of where the reference voltage is set. In the optical receiver, therefore, the reference voltage is set within a range that allows and achieves a remarkably low constant error rate (e.g., 10.sup.-12). Then, the transmission errors that do occur can be detected and corrected with an error correction code.
FIG. 5 schematically illustrates the relationship between the set range of the reference voltage and the optical signal intensity, with an error rate considered with an APD or the combination of an EDFA and PIN diode as the optical receiving element. The "1" signal indicates a wider voltage distribution than that of the "0" signal, as described above, so that the set range of the reference voltage is offset toward the "0" signal peak. When the optical signal intensity becomes lower, the set range of the reference voltage grows gradually narrower until the desired error rate cannot be achieved at all. The optical signal intensity at this point (indicated by Pmin0) is called the "minimum receiver sensitivity". Pmin0 represents the point below which the overlap of the "1" and "0" curves (which represents the error rate) is too great for the desired error rate to be achieved.
To cover applications for various transmission distances, a wide input dynamic range is required, even for a receiver for a main line. When the transmission distance is short, the optical signal attenuation through the optical fibers is so low that an intense optical signal is input to the receiver. When the transmission distance is long, on the other hand, the input optical signal is very weak. Thus, the optical receiver must be capable of handling both cases.
When a strong optical signal is input to a receiver having the construction shown in FIG. 1, the following problem, discussed with additional reference to FIGS. 6(a)-6(c), is caused by the slew rate of the preamplifier. FIGS. 6(a)-6(c) illustrate response waveforms for the receiver when a strong optical signal is input. When the intensity of the optical signal is high, the output current of the optical receiving element rises, to increasing the output swing of the preamplifier. When the output swing of the preamplifier becomes excessively large, however, a signal change occurs before the output signal reaches a steady value, as illustrated, so that the output waveform partially takes a triangular shape. The partial triangular shape results because the changing rate of the output VPRE of the preamplifier has an upper limit (dV/dt) called the "slew rate". If the slew rate of the AGC amplifier is sufficiently higher than that of the preamplifier, the output signal of the AGC amplifier has a similar waveform as that of the VPRE.
In the illustrated example, the first bit (left side) or the fifth bit (second bit from right side) are erroneously decided no matter where the reference voltage VTH(DEC) might be set, because the preferable VTH(DEC) for deciding the first bit as "0" cannot but be too high for the fifth bit to be decided as "1", and the preferable VTH(DEC) for deciding the fifth bit as "1" cannot but be too low for the first bit to be decided as "0", Vth(A). Since the output VPRE of the preamplifier has a large swing, moreover, the voltage to be applied to a transistor of the output stage of the preamplifier rises so high as to make it necesasary to use a transistor with a high breakdown voltage. Since the action rate of a transistor having a high breakdown voltage is generally lower than that of one having a low breakdown voltage, however, another problem arises in that the transistor cannot be used in a high-speed optical receiver. Thus, a highly intense optical signal is not easily handled by the construction shown in FIG. 1.
An optical receiver that is capable of avoiding these problems caused by the slew rate is disclosed on pp. 991-997 of IEEE Journal of Solid-State Circuits, Vol. 30, No. 9 (1995). FIG. 7 illustrates a construction of an optical receiver disclosed in this article. This circuit is employed in a local area network (LAN), for example, and has a transmission distance as short as several Km at most. Therefore, the APD is replaced by a PIN photodiode. The AGC amplifier is also replaced, by a limiting amplifier LA.
Although the output signal swing is held constant in the AGC amplifier by controlling the gain, the gain of the limiting amplifier LA is set at a remarkably high value so that the output signal is clamped to a predetermined voltage range, to limit the swing whenever the output swing rises above a desired value. Specifically, the reference voltage VTH(LA) is controlled by the reference voltage control circuit VCNT to be just intermediate between the high voltage (corresponding to "1", for example) and the low voltage (corresponding to "0", for example) of the input signal. The input signal and VTH(LA) are compared so that a constant voltage VLA(1) is output irrespective of the swing of the input signal if the input signal is higher than VTH(LA), but a constant voltage VLA(0) is output irrespective of the input signal swing if the input signal is lower than VTH(LA).
In this example, the gain of the limiting amplifier LA is set at about 60 dB so that an output signal of constant swing is achieved over a wide range of input signals, from several mV to about 1 V. With this construction, the gain of the limiting amplifier is so high that the gain (i.e., the ratio between the output voltage and the input current, also called the "transimpedance") of the preamplifier need not be raised so high. Even when an optical signal of high intensity is input, therefore, the swing of the output VPRE of the preamplifier is not so large as that of the aforementioned construction, so that the signal is neither distorted nor becomes a triangular wave, as illustrated in FIG. 6(b). This raises no problem due to the slew rate.
In this, however, if an APD or the combination of an EDFA and PIN diode are used as the optical receiving element instead of the PIN photodiode alone, the minimum receiver sensitivity is deteriorated. FIG. 8 illustrates a set range of the reference voltage VTH(LA) of the limiting amplifier LA in the preamplifier output VPRE. Because an optical receiving element having an amplifying function is used, the "1" signal indicates a wider voltage distribution than that of the "0" signal, and the set range of the reference voltage is offset towards the "0" signal side, as before. The output (VLA) of the limiting amplifier LA is derived by extracting a signal represented by VTH(LA).+-.several mV from the VPRE and by amplifying it (a voltage width of several mV being indicated by a double curve). The VTH(LA) is set just intermediate between the VPRE(0) and the VPRE(1), such that the VTH(LA) deviates from the set range of the reference value for a region where the optical signal intensity is Pmin1 or less. As a result, the minimum receiver sensitivity is at Pmin1, which is a lower sensitivity (i.e., Pmin1 is&gt;than Pmin0) than that of the construction of FIG. 1.
The deterioration of the minimum receiver sensitivity does not take place in the construction of this circuit, however, because the PIN diode is used as the optical receiving element. Since the PIN diode has no amplifying action, the "1" signal exhibits the same voltage distribution as that of the "0" signal, so that the aforementioned problem does not occur even if the VTH(LA) is set just intermediate between the VPRE(0) and the VPRE(1).
Thus, to avoid the problem, the reference voltage VTH(LA) of the limiting amplifier LA can be adjusted, not to the center of the VPRE, but towards the side of the "0" level. However, this method is extremely difficult to realize, because the intensity of the optical signal to be adjusted is weak (especially within a range less than Pmin1), and because the reference voltage VTH(LA) must be precisely adjusted under the condition of a small swing of the VPRE. Since the intensity of the optical signal fluctuates by the degradation in the characteristics of the transmitter or the optical fibers, it is necessary to periodically readjust the reference voltage VTH(LA). Thus, it has been impossible to use a wide dynamic range receiver in a LAN, as disclosed in this article, as has been used for long distance transmission.
Other techniques for widening the dynamic range of the optical receiver are disclosed in Japanese Patent Laid Open No. 93-259752 (Tokukai-Hei 05-259752), which discloses an improved technique for setting a reference potential of the discrimination (decision) circuit, according to which the circuit decides a voltage signal input thereto to be either "0" or "1", and in Japanese Patent Laid Open No. 96-139526 (Tokukai-Hei 08-139526), No. 95-193437 (Tokukai-Hei 07-193437), No. 95-38342 (Tokukai-Hei 07-38342), and No. 93-67930 (Tokukai-Hei 05-67930), which disclose improved circuitries for setting the gain of the preamplifier. All of these techniques are aimed to be applied for optical communication systems having a transmission rate of 100 MHz. Therefore, these techniques are expected to have sufficient responses to receive optical pulses at intervals of 10 ns, but cannot be expected to respond to optical pulses at intervals of 100 ps, as is the case for optical communication systems for trunk lines having a transmission rate of 10 GHz.
Japanese Patent Laid Open No. 97-246879 (Tokukai-Hei 09-246879), and No. 93-218758 (Tokukai-Hei 05-218758) disclose techniques for suppressing deterioration of the width of the voltage pulse in the optical receiver, but the key techniques for solving the problems illustrated in FIGS. 1 and 7, discussed above, are not disclosed.